Methylamine Separations Enabled by Cooperative Ligand Insertion in Copper-Carboxylate Metal-Organic Frameworks

Monomethylamine (NH2CH3), dimethylamine (NH(CH3)2), and trimethylamine (N(CH3)3) are important chemical feedstocks that are produced industrially as an azeotropic mixture and must be separated using an energy-intensive thermal distillation. While solid adsorbents have been proposed as alternatives to distillation for separating various industrial gas mixtures, methylamine separations remain largely unexplored in this context. Here, we investigate two isoreticular frameworks Cu(cyhdc) (cyhdc2- = trans-1,4-cyclohexanedicarboxylate) and Cu(bdc) (bdc2- = 1,4-benzenedicarboxylate) as prospective candidates for this challenging separation, motivated by the recent discovery that Cu(cyhdc) reversibly captures ammonia through a unique framework-to-coordination polymer phase change. Through a combination of gas adsorption and powder X-ray diffraction analyses, we find that Cu(cyhdc) and Cu(bdc) reversibly bind large quantities of mono- and dimethylamine through framework-to-coordination polymer phase change mechanisms, although both frameworks adsorb only moderate amounts of trimethylamine via physisorption. Single-crystal X-ray diffraction analysis of select mono- and dimethylamine containing phases suggests that the number of hydrogen bond donors available and the linker donor strength are key factors influencing amine uptake. Finally, investigation of the tricomponent adsorption behavior of both materials reveals that Cu(cyhdc) is selective for the capture of monomethylamine from a range of mono-, di-, and trimethylamine mixtures.

Exceptionally high selectivity in the separation of light hydrocarbons by adsorption on MIL-127(Fe) and on a (9,9) carbon nanotube

Porous solids with channel sizes that are not much above the size of small hydrocarbons can yield extremely large adsorption selectivity. Our Grand Canonical Monte-Carlo simulations indicate exceptionally high selectivity for the separation of methane, ethane and propane from natural gas. At 250 K the C₃H₈/CH₄ separation on MIL-127 at low pressure has a selectivity of more than 1000 and the C₃H₈/CH₄ separation on CNT (9,9) is even above 2000. This is due to the strong molecule lattice interaction in narrow channels which leads to large enthalpies of adsorption. The Arrhenius law for the Henry coefficients is analysed in order to show that the effect is due to this enthalpy rather than to steric reasons.

Adsorption and diffusion of the H2/CO2/CO/MeOH/EtOH mixture into the ZIF-7 using molecular simulation

The adsorption and diffusion of synthesis gas components (methanol, ethanol, H₂, CO_2, and CO molecules) in ZIF-7 by grand canonical Monte Carlo and molecular dynamics simulation were investigated. The initial diffusion coefficient at the beginning of the process depends on the kinetic diameter of the guest molecules. Also, the diffusion coefficient at equilibrium conditions probably depends on the interaction between the guest molecules with the ZIF-7 framework. The radial distribution function results indicate that the distribution of guest molecules in the framework is affected by the interaction between the guest molecules. These results indicate that the CO, CO₂, and H₂ guest molecules are adsorbed on both the Zn metal atom and the organic linker (especially the C₁ atom). In contrast, the organic linker is the most favorable adsorption site for methanol and ethanol guest molecules. In addition, the diffusion coefficient of guest molecules in binary mixtures is related to the attraction or repulsion between the guest molecules.

Flexible metal-organic frameworks for gas storage and separation

Flexible metal-organic frameworks (MOFs) have gradually attracted much attention due to their reversible structural changes and flexible structural responses. The basic research of flexible MOFs is to study their dynamic responses under different external stimuli and translate the responses into applications. Most research studies on flexible MOFs focus on gas storage and separation, but lack a systematic summary. Here, we review the development of flexible MOFs, the structural transformation under the external effects of temperature, pressure, and guest molecules, and their applications in gas storage and separation. Microporous MOFs with flexible structures provide unique opportunities for fine-tuning their performance because the pore shape and size can be controlled by external stimuli. The characteristics of breathing phenomena and large specific surface area make flexible MOFs suitable candidates for gas storage and separation. Finally, the application prospects of flexible MOFs are reported.

Spiers Memorial Lecture: Coordination networks that switch between nonporous and porous structures: an emerging class of soft porous crystals

Coordination networks (CNs) are a class of (usually) crystalline solids typically comprised of metal ions or cluster nodes linked into 2 or 3 dimensions by organic and/or inorganic linker ligands. Whereas CNs tend to exhibit rigid structures and permanent porosity as exemplified by most metal-organic frameworks, MOFs, there exists a small but growing class of CNs that can undergo extreme, reversible structural transformation(s) when exposed to gases, vapours or liquids. These "soft" or "stimuli-responsive" CNs were introduced two decades ago and are attracting increasing attention thanks to two features: the amenability of CNs to design from first principles, thereby enabling crystal engineering of families of related CNs; and the potential utility of soft CNs for adsorptive storage and separation. A small but growing subset of soft CNs exhibit reversible phase transformations between nonporous (closed) and porous (open) structures. These "switching CNs" are distinguished by stepped sorption isotherms coincident with phase transformation and, perhaps counterintuitively, they can exhibit benchmark properties with respect to working capacity (storage) and selectivity (separation). This review addresses fundamental and applied aspects of switching CNs through surveying their sorption properties, analysing the structural transformations that enable switching, discussing structure-function relationships and presenting design principles for crystal engineering of the next generation of switching CNs.

Selective guest inclusion of linear C6 hydrocarbons in a Zn(ii) 1D coordination polymer

Trapping of volatile unbranched C₆ hydrocarbons (hexane, 1-hexene, and 1-hexyne) in a 1D coordination polymer is reported.

Tracking the dimensional conversion process of semiconducting lead bromide perovskites by mass spectroscopy, powder X-ray diffraction, microcalorimetry and crystallography

The structural information of a material in both the solid state and solution state is essential to the in-depth understanding of the properties of inorganic-organic hybrid materials. A one-dimensional (1D) lead bromide formulated as [H][NH₃(CH₂)₂SS(CH₂)₂NH₃][H₂O][PbBr₅] (1) could be converted into a new two-dimensional (2D) complex, [NH₃(CH₂)₂SS(CH₂)₂NH₃][PbBr₄] (2), by soaking the crystals in water. The isolated 2D compound showed single-layer lead-halide perovskite structures. Electrospray ionization mass spectrometry (ESI-MS) analyses of the reaction solution revealed that the [PbBr₃]^- fragments are initially formed from the rapid decomposition of the 1D [PbBr₅]^3- chains and subsequently reassemble into 2D [PbBr₄]^2- layers, which was verified by powder X-ray diffraction (PXRD) and microcalorimetry. Because of the decomposition and reassembly process, complex 1 could be used as a precursor to synthesize M²⁺-doped 2D lead bromide perovskites, namely, Mn@2, Ni@2 and Cd@2. In addition, preliminary tests indicated that complex 2 exhibited a lower optical band gap (3.25 eV) and higher electrical conductivity (3.2 × 10^-11 S cm^-1) than complex 1 (3.38 eV, 5.4 × 10^-12 S cm^-1).

Desolvation process in the flexible metal–organic framework [Cu(Me-4py-trz-ia)], adsorption of dihydrogen and related structure responses

Structural changes and the unusual H₂ adsorption behaviour of a Cu²⁺-based MOF were studied by X-ray diffraction in combination with DFT modelling and by inelastic neutron scattering.

Extension of the QuickFF force field protocol for an improved accuracy of structural, vibrational, mechanical and thermal properties of metal-organic frameworks

QuickFF was originally launched in 2015 to derive accurate force fields for isolated and complex molecular systems in a quick and easy way. Apart from the general applicability, the functionality was especially tested for metal-organic frameworks (MOFs), a class of hybrid materials consisting of organic and inorganic building blocks. Herein, we launch a new release of the QuickFF protocol which includes new major features to predict structural, vibrational, mechanical and thermal properties with greater accuracy, without compromising its robustness and transparent workflow. First, the ab initio data necessary for the fitting procedure may now also be derived from periodic models for the molecular system, as opposed to the earlier cluster-based models. This is essential for an accurate description of MOFs with one-dimensional metal-oxide chains. Second, cross terms that couple internal coordinates (ICs) and anharmonic contributions for bond and bend terms are implemented. These features are essential for a proper description of vibrational and thermal properties. Third, the fitting scheme was modified to improve robustness and accuracy. The new features are tested on MIL-53(Al), MOF-5, CAU-13 and NOTT-300. As expected, periodic input data are proven to be essential for a correct description of structural, vibrational and thermodynamic properties of MIL-53(Al). Bulk moduli and thermal expansion coefficients of MOF-5 are very accurately reproduced by static and dynamic simulations using the newly derived force fields which include cross terms and anharmonic corrections. For the flexible materials CAU-13 and NOTT-300, the transition pressure is accurately predicted provided cross terms are taken into account. © 2018 Wiley Periodicals, Inc.

Structure and properties of dynamic metal–organic frameworks: a brief accounts of crystalline-to-crystalline and crystalline-to-amorphous transformations

External stimuli-driven structural changes and the associated properties of dynamic MOFs are discussed with examples.

Methane Adsorption in Zr-Based MOFs: Comparison and Critical Evaluation of Force Fields

The search for nanoporous materials that are highly performing for gas storage and separation is one of the contemporary challenges in material design. The computational tools to aid these experimental efforts are widely available, and adsorption isotherms are routinely computed for huge sets of (hypothetical) frameworks. Clearly the computational results depend on the interactions between the adsorbed species and the adsorbent, which are commonly described using force fields. In this paper, an extensive comparison and in-depth investigation of several force fields from literature is reported for the case of methane adsorption in the Zr-based Metal-Organic Frameworks UiO-66, UiO-67, DUT-52, NU-1000, and MOF-808. Significant quantitative differences in the computed uptake are observed when comparing different force fields, but most qualitative features are common which suggests some predictive power of the simulations when it comes to these properties. More insight into the host-guest interactions is obtained by benchmarking the force fields with an extensive number of ab initio computed single molecule interaction energies. This analysis at the molecular level reveals that especially ab initio derived force fields perform well in reproducing the ab initio interaction energies. Finally, the high sensitivity of uptake predictions on the underlying potential energy surface is explored.

Fine-Tuning of the Carbon Dioxide Capture Capability of Diamine-Grafted Metal-Organic Framework Adsorbents Through Amine Functionalization

A combined sonication and microwave irradiation procedure provides the most effective functionalization of ethylenediamine (en) and branched primary diamines of 1-methylethylenediamine (men) and 1,1-dimethylethylenediamine (den) onto the open metal sites of Mg₂ (dobpdc) (1). The CO₂ capacities of the advanced adsorbents 1-en and 1-men under simulated flue gas conditions are 19 wt % and 17.4 wt %, respectively, which are the highest values reported among amine-functionalized metal-organic frameworks (MOFs) to date. Moreover, 1-den exhibits both a significant working capacity (12.2 wt %) and superb CO₂ uptake (11 wt %) at 3 % CO₂ . Additionally, this framework showcases the superior recyclability; ultrahigh stability after exposure to O₂ , moisture, and SO₂ ; and exceptional CO₂ adsorption capacity under humid conditions, which are unprecedented among MOFs. We also elucidate that the performance of CO₂ adsorption can be controlled by the structure of the diamine ligands grafted such as the number of amine end groups or the presence of side groups, which provides the first systematic and comprehensive demonstration of fine-tuning of CO₂ uptake capability using different amines.

Synthesis of a partially fluorinated ZIF-8 analog for ethane/ethene separation

ZIF-318, isostructural to ZIF-8 but built from the mixed linkers of 2-methylimidazole and 2-trifluoromethylimidazole can be activated for gases sorption and the separation of ethane/ethene mixtures.

Metal-Organic Frameworks as a Potential Platform for Selective Treatment of Gaseous Sulfur Compounds

The release of various anthropogenic pollutants such as gaseous sulfur compounds into the environment has been accelerated as globalization has promoted the production of high-quality products at lower prices. Because of strict enforcement of mitigation technologies, advanced materials have been developed to efficiently remove gaseous sulfur compounds released from various source processes. Metal-organic frameworks (MOFs) are promising materials to treat sulfur compounds via adsorption, catalysis, or separation. Nonetheless, the practical applicability of MOFs is limited by a number of factors including loss of structural integrity after use, limited reusability of spent MOFs, and low stability toward omnipresent molecules (e.g., H₂O). Here, we provide a comprehensive assessment of MOF technology for the effective control of gaseous sulfur compounds. This review will thus help expand the fields of real-world application for MOFs with a roadmap for this highly challenging area of research.

Structural flexibility in crystallized matter: from history to applications

The large reversible flexibility of hybrid crystallized matter is relatively new. After briefly recalling the history of this discovery, the article will analyze the different parameters influencing this phenomenon. They relate first to the various structural characteristics of the framework, in both its inorganic and organic parts. The influence of the energies of the guest-guest and host-guest interactions is then analyzed. Once the reasons are explained, a third section will be devoted to the various physical properties of these flexible solids. The last section concerns recent industrial applications of this family of solids.

Paddle Wheel Based Triazolyl Isophthalate MOFs: Impact of Linker Modification on Crystal Structure and Gas Sorption Properties

Syntheses and comprehensive characterization of two closely related series of isomorphous metal-organic frameworks (MOFs) based on triazolyl isophthalate linkers with the general formula ∞(3)[M2(R(1)-R(2)-trz-ia)2] (M = Cu, Zn) are presented. Using solvothermal synthesis and synthesis of microcrystalline materials on the gram scale by refluxing a solution of the starting materials, 11 MOFs are readily available for a systematic investigation of structure-property relationships. The networks of the two series are assigned to rutile (rtl) (1-4) and α-PbO2 (apo) (5-9) topology, respectively. Due to the orientation of the triazole substituents toward the cavities, both the pore volume and the pore diameter can be adjusted by choice of the alkyl substituents. Compounds 1-9 exhibit pronounced microporosity with calculated porosities of 31-53% and show thermal stability up to 390 °C as confirmed by simultaneous thermal analysis. Systematic investigation of adsorption properties by CO2 (298 K) and N2 (77 K) adsorption studies reveal remarkable network flexibility induced by alkyl substituents on the linker. Fine-tuning of the gate opening pressure and of the hysteresis shape is possible by adjusting the substitution pattern and by choice of the metal ion.

Giant flexibility of crystallized organic–inorganic porous solids: facts, reasons, effects and applications

Giant structural flexibility is a characteristic of organic–inorganic frameworks. This perspective describes its history, its behaviours, the analysis of its structural reasons at its consequences in terms of properties and applications.

Adsorptive Separation of Olefin/Paraffin Mixtures with ZIF-4

The microporous zeolitic imidazolate framework ZIF-4 has been synthesized, and its ethylene/ethane and propylene/propane separation potentials have been evaluated by single-component adsorption isotherms and breakthrough experiments of the respective binary mixtures. In all experiments, a higher selectivity for the paraffin is observed that is manifested by a steeper equilibrium isotherm as well as a later breakthrough in the fixed-bed adsorber experiments. Microporous adsorbents with paraffin selectivity are rare but highly interesting for cyclic adsorption processes such as pressure-swing adsorption (PSA).

Tolerance of Flexible MOFs toward Repeated Adsorption Stress

The adsorption/desorption cyclability of four flexible MOFs, namely, MIL-53(Al), ELM-11, DUT-8(Ni), and SNU-9, was studied at 298 K using n-butane as adsorptive. The detailed analysis of thermal response curves, physisorption isotherm data, powder X-ray diffraction patterns, as well as SEM images revealed the highly stable switching performance of MIL-53(Al) and ELM-11 materials during 100 adsorption/desorption cycles. In contrast, for DUT-8(Ni) and SNU-9, the multiple adsorption/desorption stress leads to the reduction of crystallite size, causing changes in the switching behavior in the initial 10 physisorption runs, and a characteristic shift of the "gate-opening" pressure to higher values is observed.

DFT-Derived Force Fields for Modeling Hydrocarbon Adsorption in MIL-47(V)

Generic force fields such as UFF and DREIDING are widely used for predicting molecular adsorption and diffusion in metal-organic frameworks (MOFs), but the accuracy of these force fields is unclear. We describe a general framework for developing transferable force fields for modeling the adsorption of alkanes in a nonflexible MIL-47(V) MOF using periodic density functional theory (DFT) calculations. By calculating the interaction energies for a large number of energetically favorable adsorbate configurations using DFT, we obtain a force field that gives good predictions of adsorption isotherms, heats of adsorption, and diffusion properties for a wide range of alkanes and alkenes in MIL-47(V). The force field is shown to be transferable to related materials such as MIL-53(Cr) and is used to calculate the free-energy differences for the experimentally observed phases of MIL-53(Fe).

Adsorption and Separation of Small Hydrocarbons on the Flexible, Vanadium-Containing MOF, COMOC-2

COMOC-2, a flexible vanadium-containing metal-organic framework, was investigated for its adsorption and separation properties of light hydrocarbons. COMOC-2 is an extended version of the MIL-47 framework with 4,4'-biphenyldicarboxylic acid linkers instead of terephthalic acid. Adsorption isotherms of methane to propane, ethylene, and propylene were determined with a gravimetric uptake technique at temperatures between 281 and 303 K. A pronounced breathing effect was observed (in contrast to the more rigid MIL-47 framework) in which the adsorption capacity increases by more than a factor of 2 at a given breathing pressure. The breathing pressure decreases with increasing hydrocarbon molecular weight. The typical two-step isotherms are nearly identical for alkanes and alkenes, in accordance with the nonpolar nature of the material. Binary isotherms of ethane and propane were also measured with the gravimetric uptake technique at different temperatures and total pressures. The mixture isotherms and breathing transition pressures were predicted by relying on the osmotic framework adsorbed solution theory (OFAST). Finally, the separation potential of COMOC-2 for ethane/propane mixtures was looked into using breakthrough experiments for different compositions and different pressures.

MFI and FAU-Type Zeolites as Trapping Materials for Light Hydrocarbons Emission Control at Low Partial Pressure and High Temperature

The adsorption of light hydrocarbons (C2–C5 olefins and paraffins, toluene) on HZSM-5, silicalite, and HY was studied for application in treatment of exhaust streams of the petrochemical industry and of vehicles under cold start conditions. At this aim the trapping capability was evaluated on hydrated zeolites by breakthrough curves at low hydrocarbon partial pressure (0-1 kPa), in the temperature range 298–523 K and at space velocity of 30000 h−1. The basic adsorption properties of materials were also verified for three selected hydrocarbons (ethylene, isobutene, and toluene) by equilibrium isotherms on dehydrated zeolites at 298 K. The role of physicochemical characteristics of adsorbent materials was discussed in relation with their trapping capability of different types of hydrocarbons.

Crystallographic studies of gas sorption in metal-organic frameworks

Metal-organic frameworks (MOFs) are a class of porous crystalline materials of modular design. One of the primary applications of these materials is in the adsorption and separation of gases, with potential benefits to the energy, transport and medical sectors. In situ crystallography of MOFs under gas atmospheres has enabled the behaviour of the frameworks under gas loading to be investigated and has established the precise location of adsorbed gas molecules in a significant number of MOFs. This article reviews progress in such crystallographic studies, which has taken place over the past decade, but has its origins in earlier studies of zeolites, clathrates etc. The review considers studies by single-crystal or powder diffraction using either X-rays or neutrons. Features of MOFs that strongly affect gas sorption behaviour are discussed in the context of in situ crystallographic studies, specifically framework flexibility, and the presence of (organic) functional groups and unsaturated (open) metal sites within pores that can form specific interactions with gas molecules.

Adsorption performance of a MIL-101(Cr)/graphite oxide composite for a series of n-alkanes

A novel composite MIL-101@GO based on MIL-101(Cr) and graphite oxide (GO) shows high adsorption capacities and excellent adsorption–desorption performance for a series of n-alkanes.

Pulse gas chromatographic study of adsorption of substituted aromatics and heterocyclic molecules on MIL-47 at zero coverage

The low coverage adsorptive properties of the MIL-47 metal organic framework toward aromatic and heterocyclic molecules are reported in this paper. The effect of molecular functionality and size on Henry adsorption constants and adsorption enthalpies of alkyl and heteroatom functionalized benzene derivates and heterocyclic molecules was studied using pulse gas chromatography. By means of statistical analysis, experimental data was analyzed and modeled using principal component analysis and partial least-squares regression. Structure-property relationships were established, revealing and confirming several trends. Among the molecular properties governing the adsorption process, vapor pressure, mean polarizability, and dipole moment play a determining role.

Complexity behind CO2 capture on NH2-MIL-53(Al)

Some Metal Organic Frameworks (MOFs) show excellent performance in extracting carbon dioxide from different gas mixtures. The origin of their enhanced separation ability is not clear yet. Herein, we present a combined experimental and theoretical study of the amino-functionalized MIL-53(Al) to elucidate the mechanism behind its unusual high efficiency in CO(2) capture. Spectroscopic and DFT studies point out only an indirect role of amine moieties. In contrast to other amino-functionalized CO(2) sorbents, no chemical bond between CO(2) and the NH(2) groups of the structure is formed. We demonstrate that the functionalization modulates the "breathing" behavior of the material, that is, the flexibility of the framework and its capacity to alter the structure upon the introduction of specific adsorbates. The absence of strong chemical interactions with CO(2) is of high importance for the overall performance of the adsorbent, since full regeneration can be achieved within minutes under very mild conditions, demonstrating the high potential of this type of adsorbents for PSA like systems.